We have designed the backward wave oscillator, a power-pulsed generator oscillated at 20 GHz has higher frequency than current one. An absolute instability linear analysis was used for the purpose of designing the slow wave structure. A large diameter (D/$lambda$=4.8) of the slow wave structure was adopted to prevent the breakdown brought about by the increase of power density. We have fabricated a marx generator, pulse forming line and diode. And the development of a compact pulsed power generator with short period and low amplitude is expected.